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Homework Help: Converging Lens and Sun

  1. Mar 25, 2010 #1


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    1. The problem statement, all variables and given/known data
    A camper is trying to start a fire by focusing sunlight onto a piece of paper. The diameter of the sun is 1.39e9 m, and its mean distance from the earth is 1.50e11 m. The camper is using a converging lens whose focal length is 10.8 cm.
    (a) What is the area of the sun's image on the paper?
    (b) If 0.568 W of sunlight passes through the lens, what is the intensity of the sunlight at the paper?

    2. Relevant equations
    (a) A=.25*pi*(height of sun's image)^2 and m=-di/do
    (b) I=P/A

    3. The attempt at a solution
    B should not be an issue if I can find A. I do not know how to find the height of the sun's image.
  2. jcsd
  3. Mar 25, 2010 #2


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    Try 1/f = 1/do + 1/di

    This is the "thin lens" formula. You'll use it a lot when studying lenses. You might want to commit it to memory.
  4. Mar 25, 2010 #3


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    Thanks for the reply! I really appreciate your quick response! :smile: That is one of the main formulas that we use.

    I managed to solve the problem by taking the focal length over the distance earth to sun and then multiplying that by the diameter of the sun.
    (.108/ 1.5E-11) * 1.39E9= h
    then taking pi/4 * h^2 =A

    My problem before was that I was using 10.8 instead.
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